Interventional cardiology, interventional angiology and other interventional techniques in cardiovascular and other vessels, ducts and channels of the human body have demonstrated marked success in recent years. Studies of interventions in the treatment of acute myocardial infarction (MI), for example, indicate the effectiveness of primary angioplasty. Implantation of coronary stents has improved the outcome of such interventional treatment. For example, these results are described in an article in the Journal of American College of Cardiology 2000, 1194-1201.
Stents are being implanted in increasing numbers throughout the world to treat heart and cardiovascular disease, and are also coming into greater use outside strictly the field of cardiology. For example, other vascular interventions utilizing stents which are proving to be of equal importance to use in cardiology include stenting of the carotid, iliac, renal, and femoral arteries. Moreover, vascular intervention with stents in cerebral circulation is exhibiting quite promising results, especially in patients suffering acute stroke.
Stents are implanted in vessels, ducts or channels of the human body to act as a scaffolding to maintain the patency of the vessel, duct or channel lumen. A drawback of stenting is the body's natural defensive reaction to the implant of a foreign object. In many patients, the reaction is characterized by a traumatic proliferation of tissue as intimal hyperplasia at the implant site, and, where the stent is implanted in a blood vessel such as a coronary artery, formation of thrombi which become attached to the stent. Each of these adverse effects contributes to restenosis—a re-narrowing of the vessel lumen—to compromise the improvements that resulted from the initial re-opening of the lumen by implanting the stent. Consequently, a great number of stent implant patients must undergo another angiogram, on average about six months after the original implant procedure, to determine the status of tissue proliferation and thrombosis in the affected lumen. If re-narrowing has occurred, one or more additional procedures are required to stem or reverse its advancement.
For virtually all stent implant patients it is desirable to examine and analyze the patency of the vessel lumen and the extent of tissue growth within the lumen of the stent, and to measure blood flow therethrough, from time to time as part of the patient's routine post-procedure examinations. Current techniques employed to analyze patency of the lumen following a stent implant procedure are more or less invasive.
Among these techniques is vascular puncture, which, despite a relatively low complication rate, poses inherent risks as well as discomfort of the patient, such as a need for compression of the puncture site. Use of iodine containing contrast dye also presents the possibility of negative implication such as renal failure, especially in patients with diabetes. If contrast dyes are applied to a cerebral perfusion, tissue damage may cause neurological seizures and temporary cerebral dysfunction. Therefore, it is advantageous to determine the vascular status and the functional and morphological capacity of the vascular bed by less or non-invasive methods, including methods not requiring application of iodine containing contrast dye.
Fluoroscopic techniques are an unsuitable substitute or alternative for the invasive methods because the metal stent itself causes blockage of the x-rays. Although visualization of the stent is achieved by its fluoroscopic portrayal as a shadow during the original implant procedure, the stent's very presence defeats subsequent examination of the interior condition of the stent and the vessel lumen at the implant site by means of fluoroscopy following the implant procedure, without the use of contrast dye applied intravascularly.
Magnetic resonance imaging (MRI) can be used to visualize internal features of the body if there is no magnetic resonance distortion. MRI has an excellent capability to visualize the vascular bed, with particularly accurate imaging of the vascular structure being feasible following the application of gadolinium, a contrast dye which enhances the magnetic properties of the blood and which stays within the vascular circulation. This has special implications for the perfusion in vessels which are in a stable and resting state, especially iliac, femoral, carotid, and cerebral perfusion. On occasion of acute cerebrovascular stroke, the diagnosis of a blocked artery can be achieved quickly, within minutes, by means of an MRI technique following the intravenous injection of 30 milliliters (ml) of gadolinium.
Imaging procedures using MRI without need for contrast dye are emerging in the clinical practice. But a current considerable factor weighing against the use of magnetic resonance imaging techniques to visualize implanted stents composed of ferromagnetic or electrically conductive materials is the inhibiting effect of such materials. These materials cause sufficient distortion of the magnetic resonance field to preclude imaging the interior of the stent. This effect is attributable to their Faradaic physical properties in relation to the electromagnetic energy applied during the MRI process.
It is a primary aim of the present invention to provide a stent structure and method that enables imaging and visualization of the inner lumen of an implanted stent by means of an MRI technique without need for X-ray or contrast dye application. A related aim is to enable analysis and evaluation of the degree of tissue proliferation and thrombotic attachment within the stent, and thereby, calculation of the extent of restenosis within the stent, as well as to measure the degree of blood flow, using only MRI and electromagnetic measurement of blood flow.
In German application 197 46 735.0, which was filed as international patent application PCT/DE98/03045, published Apr. 22, 1999 as WO 99/19738, Melzer et al (Melzer, or the 99/19738 publication) disclose an MRI process for representing and determining the position of a stent, in which the stent has at least one passive oscillating circuit with an inductor and a capacitor. According to Melzer, the resonance frequency of this circuit substantially corresponds to the resonance frequency of the injected high-frequency radiation from the magnetic resonance system, so that in a locally limited area situated inside or around the stent, a modified signal answer is generated which is represented with spatial resolution. However, the Melzer solution lacks a suitable integration of an LC circuit within the stent.
Therefore, it is another significant aim of the present invention to provide a structure which enhances the properties of the stent itself to allow MRI imaging within the interior of the lumen of the implanted stent.